I am the mum of a child born following a one-night stand, who has never met the bio-dad. Current DH is her dad in every other way, and she knows the basics about bio-dad.
Another thread made me wonder, how does it feel to be the wife/partner of someone who has another child he doesn't see? Especially those who have never met their offspring, like our situation, or who happily don't see them. Is it weird? Do you wonder about your children's half-sibling that they don't know? Do you want to meet them?
I must admit to googling my child's bio-dad, and have worked out that he has other kids and lives in the same state, but he made it quite clear at the start that he never wanted "a knock on the door in 18 years" so we have never looked into contact or anything, and are quite happy with things as they are.